Extract common file download and audio detection logic to utils package,
implement consistent temp file cleanup with defer, add allowlist checks
before downloading attachments, and improve context management across
Discord, Slack, and Telegram channels. Replace logging with structured
logger and prevent context leaks in transcription and thinking animations.
Multiple packages had their own private truncate implementations:
- channels/telegram.go: truncateString (byte-based, no "...")
- channels/dingtalk.go: truncateStringDingTalk (byte-based, no "...")
- voice/transcriber.go: truncateText (byte-based, with "...")
All three are functionally equivalent to the existing utils.Truncate,
which already handles rune-safe truncation and appends "..." correctly.
Replace all private copies with utils.Truncate and delete the dead code.
